In recent years, position input apparatuses have been used as input devices for tablet type personal computers (PCs) and the like. A position input apparatus includes a position pointing device which is formed, for example, to be of a pen type, and a position detecting device having an input surface on which a pointing operation and inputting of characters, figures or the like are conducted using the position pointing device.
As a pen-type position pointing device of this kind, conventionally, those for use with a position detecting device based on an electromagnetic induction system have been widely known. The position pointing device of the electromagnetic induction system has a resonance circuit configured by connecting a resonant capacitor to a coil wound around a magnetic-material core, for example, a ferrite core. The position pointing device transmits a resonance signal, obtained by the resonance circuit, to the position detecting device, thereby pointing a position to the position detecting device.
In addition, the pen type position pointing device of this kind has conventionally been configured to have a function to detect a pressure (writing pressure) exerted on the leading end part (pen point) of the core body and to transmit the pressure to the position detecting device. In this case, for detection of the writing pressure, there has been known a method of using a mechanism by which the capacitance of a capacitor constituting the resonance circuit or the inductance of a coil constituting the resonance circuit is varied according to the writing pressure.
FIG. 18 illustrates a configuration example of a conventional position pointing device including a writing pressure detection module of a variable capacitor type wherein the capacitance of a capacitor constituting a resonance circuit of the position pointing device is varied according to the writing pressure. This example of position pointing device is described in Patent Document 1 (Japanese Patent Laid-Open No. 2011-186803). FIG. 18 is a longitudinal sectional view for explaining the configuration example of the position pointing device.
As shown in FIG. 18, a position pointing device 100 includes a core body 102, a position pointing coil 104 wound around a ferrite core 103, a variable capacitor 105, and a printed circuit board 106, which are disposed inside a tubular casing (case) 101.
The casing 101 is in a cylindrical shape, having an opening 101a on one end in the axial direction and closed on the other end in the axial direction. In a position near the other end in the axial direction inside a cylindrical cavity of the casing 101, the printed circuit board 106 with electronic components mounted thereon is fixed by securing means such as adhesive or screw. The ferrite core 103 is housed in the casing 101 in a position near the one end where the opening 101a is formed.
The ferrite core 103 is in the shape of, for example, a circular cylinder provided therein with a penetration hole 103a in the axial direction, and the position pointing coil 104 constituting a resonance circuit is wound on the periphery of the ferrite core 103. Both ends (not shown) of the position pointing coil 104 are electrically connected to electronic components constituting the resonance circuit which is mounted on the printed circuit board 106. The core body 102 is inserted in and extends through the penetration hole 103a of the ferrite core 103.
The core body 102 is a bar-shaped member having a substantially constant thickness (radial size), and is formed of an elastic material so as not to mar an input surface composed of a glass surface or the like. One end in the axial direction of the core body 102 is a leading end part 102a serving as a pen point. In the example depicted in FIG. 18, the one end is formed in a roughly conical shape. When the core body 102 is housed inside the casing 101 by inserting it in the penetration hole 103a of the ferrite core 103, the one end protrudes from the opening 101a to be exposed to the exterior. Other end 102b in the axial direction of the core body 102 is connected to the variable capacitor 105.
Because the leading end part 102a is brought into contact with a display screen or the like, the core body 102 is formed of, for example, an elastic resin such as not to mar the display screen or the like.
Although a detailed drawing is omitted in FIG. 18, the variable capacitor 105 has a configuration wherein a first electrode and a second electrode are arrayed in the axial direction, with a circular disc-shaped dielectric interposed therebetween. The second electrode is preliminarily attached to a flat surface, on the printed circuit board 106 side, of the dielectric. The first electrode is composed of a conductor which is formed of an elastic material and a tip surface of which is in a spindle-like shape. The first electrode is provided on the core body 102 side, with its spindle-like tip surface opposed to the dielectric. The other end 102b side of the core body 102 is connected to the conductor constituting the first electrode.
When a pressure in the axial direction is exerted on the leading end part 102a of the core body 102, the conductor formed of the elastic material constituting the first electrode is pressed against the dielectric by the pressure, whereby the conductor is elastically deformed, and the area of contact between the dielectric and the conductor is varied. The variable capacitor 105 exhibits a capacitance according to the area of contact. In other words, the variable capacitor 105 has a capacitance according to the pressure exerted on the core body 102. Since the variable capacitor 105 constitutes part of the resonance circuit, by transmitting the resonance frequency of the resonance circuit from the position pointing device 100 to the position detecting device side, the writing pressure exerted on the core body 102 of the position pointing device 100 can be detected on the side of the position detecting device.